Qualitative Analysis of Carbohydrates (All tests)

Carbohydrates, also called hydrated carbons are the most abundant organic compounds in human body. They contain hydrogen, oxygen and carbons. They are further classified into monosaccharides, disaccharides and polysaccharides.

Monosaccharides:

They are the simplest sugars/carbohydrates. It means that the cannot be further hyrdolysed to smaller sugars. Some examples of these sugars include: glucose, galactose, frutose etc.

Disaccharides:

They are compound sugars and made up of two monosacchardies like sucrose, maltose etc.

Polycassharides:

They are complex sugars that are made from the combination of many simple sugars. They from different structures of human body. Eg cellulose and starch etc.

How To Find Carbohydrates In Solution?

After learning the basics about carbohydrates, it is important for a chemistry or biochemistry student to have some knowledge of qualitative analysis of carbohydrates.

Qualitative analysis of carbs means that finding out which specific sugar is present in a given unknown solution. This can be done by performing some simple tests on an unknown solution.

We have already covered most of the these tests in other posts (links given below with each test). But this post if for the general precedure and sequence in which you should carry out these tests so that you can easily find the carbohydrate in given solution. If you follow the sequence given below, you would not have to perform all the tests. Means that you would save time and reagents during your analysis. For further info about procedures etc of these separate tests, you should follow the links to separate posts of these tests.

Qualitative Tests of Carbohydrates:

In above paragraphs, i have already explained the reasons why you would perform test for qualitative analysis. Now lets learn about those tests that you have to perform:

1. Moslish’s Test:

This is the first test that you must perform during qualitative analysis of carbs. The reason is that, if you are a given a specific solution for determination of some specific sugar in it, but that solution doesn’t contain any sugar at all you would waste your time, energy and reagents performing other tests if you don’t perform moslish test first.

So, molisch’s test being the general test for carbs must be performed first if don’t know whether carbs are present in your solution or not.

As mentioned, this test is general test for carbohydrates. It means that this test will tell you whether you solution contain carbohydrate or not. It could be any carbohydrate. This test would be positive for all carbs i.e mono, di and polysaccharides.

We have written a compete detailed post on molisch test that includes its principle, reagents, procedure, precautions and results. You can learn about it here.

2. Fehling’s Test:

This is the second test you should perform during your qualitative analysis. This test will tell you whether your solution contains a reducing sugar or non reducing sugar.

Reducing Sugars:

Those sugars that have a free aldehyde or ketone group  can act as a reducing agent and hence they are called reducing sugars.

Almost all monosaccharides i.e glucose, galactose and fructose are reducing sugars.

While disaccharides can be reducing or non reducing. For example lactose and maltose are reducing sugars while sucrose is non reducing sugar.

Non Reducing Sugars:

These don’t have free ketone or aldehyde and thus are not reducing agents. Like sucrose.

As mentioned above by performing the fehling’s test, you can find whether a sugar present in unknown solution is reducing or not. After finding this, you can easily find out which sugar is present by performing few other simple tests.

3. Benedict’s Test:

This test is the same as fehling test. Which means that this test will also tell you whether an unknown solution contains reducing or non reducing carbohydrate. But the difference is that, this test can be also used for quantitative analysis. So this test is superior to fehling’s test.

We have a written a separate post on benedict’s test along with its procedure, principle, precautions and reagents etc. You should read it to perform this test.

4. Barfoed’s Test:

This test is to rule out monosaccharides from disaccharides and polysaccharides. Means if you have an unknown solution and you have already done moslisch’s test, it means that the solution contains carbs. After this if you have done benedict’s test it and it contains reducing sugars than it means the solution could contain monosaccharides or some disaccharides like maltose or lastose.

So after performing barfoard’s test, if it is positive it means that your unknow solutions contains monosaccharides.

Monosaccharides react fast and precipitate forms within 7 minutes. So if precipitate forms within seven minutes, it means that the solution contains monosaccharides.

I have written a separate post on barfoed’s test in which its principle, reagents, precudure, precautions and results are given. You can click the link above to read it there.

5. Saliwanoff’s Test:

After performing barfeod’s test if it is positive and you have sure that you solution contain monosaccharides, you can further investigate to find which monosaccharide is it.

In case of saliwanoff’s test, it would tell you whether your solution contains fructose or not. If this test is positive, it means there is keto sugar present in your solution.

So if you have positive barfoed’s test and also positive saliwanoff’s test, it means that the unknown solutions contains fructose.

And if you have negative barfoed’s test and positive saliwanoff’s test, it means that the unknown solution that you have will contain sucrose.

If you are interested to learn how to perform saliwanoff test, read our separate post on that.

6. Bail’s Test:

It is another test to find whether a monosaccharide contain pentose sugar or hexose sugar. If you have a positive Barfoed’s test, means if you know that the unkown solution contains monosaccharides, you can further investigate to find whether that monosaccharide is pentose or hexose. For this you would have to perform bail’s test.

We have written a detailed post on Bail’s test and its principle and procedure etc. To learn more about it in depth read that post.

7. Iodine Test:

If barfoed’s reagent react very slow or not at all it means that the sugar contained in an unknown solution is polysaccharide. Now if you want to further investigate that which polysaccharide is present in it, you can do so by performing Iodine Test. This test is positive for starch, means that you can detect the presence of starch using this simple test.

You can read in detail about the principle, reagents, procedure and precautions of Iodine Test here.

8. Osazone Test:

Now this test is one of the most important test. The reason is that it can tell you about a lot of sugars in just one test.

You can know whether the solution contains glucose, fructose and mannose because all of them give needle shape osazone crystals. Same is the case with lactose, which produce mushroom shaped crystals and Flower-shaped crystals are produced by maltose.

It means that different sugars give different shape of crystals which can be seen under microscope.

Qualitative Analysis of Carbohydrates pdf:

If you want to learn about carbs qualitative analysis from an article that is in pdf format, you can do so by downloading the pdf copy below.

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